GaN related compound semiconductor and process for producing the same

ABSTRACT

A layer comprising cobalt (Co) is formed on a p +  layer by vapor deposition, and a layer comprising gold (Au) is formed thereon. The two layers are alloyed by a heat treatment to form a light-transmitting electrode. The light-transmitting electrode therefore has reduced contact resistance and improved light transmission properties, and gives a light-emitting pattern which is stable over a long time. Furthermore, since cobalt (Co) is an element having a large work function, satisfactory ohmic properties are obtained.

This is a continuation of application Ser, No. 09/819,622, which wasfiled on Mar. 29, 2001, which is a divisional of application Ser. No.08/979,346, which was filed on Nov. 26, 1997, now U.S. Pat. No.6,291,840, the contents of both of which are incorporated by referencein their entirety.

This application claims foreign priority from Japanese applications Hei.8-334956 filed Nov. 29, 1996; Hei. 9-19748 filed Jan. 17, 1997; and Hei.9-47064 filed Feb. 14, 1997, all of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device having a light-transmittingelectrode and a pad electrode which are formed on a p-type GaN relatedcompound semiconductor layer.

2. Description of the Related Art

In conventional compound semiconductors, an ohmic contact is obtained bydepositing metals on the semiconductor surface and heating the metals toconvert the same to an alloy and to cause metal diffusion into thesemiconductor, because an ohmic contact is not obtainable by the meredeposition of metals.

Even when p-type GaN related compound semiconductors are subjected to atreatment for reducing resistance, e.g., irradiation with electronbeams, the thus-treated semiconductors still have higher resistivitiesthan n-type GaN related compound semiconductors. Consequently, in suchp-type GaN related compound semiconductors, the p-type layer has almostno current flow in lateral directions, and only the part thereofdirectly beneath the electrode emitts light.

Under these circumstances, a current-diffusing electrode having lighttransmission properties and ohmic properties has been proposed which isformed by depositing a nickel (Ni) layer and a gold (Au) layer, eachhaving a thickness of several tens of angstroms, (Å) and heating themetal layers (see Japanese Unexamined Patent Publication No. Hei.6-314822).

However, the electrode formed by depositing nickel (Ni) and gold (Au)each having a thickness of several tens of angstroms and heating themetals poses a problem that the light-emitting pattern qualitydeteriorates with the lapse of time, resulting in an increased drivingvoltage. However, the electrode has satisfactory optical and electricalcharacteristics in the initial stage.

The reason for the quality deterioration is believed to be as follows.Since the nickel (Ni) and gold (Au) deposited layers are extremely thin,part of the nickel (Ni) is replaced by gold (Au) during the heattreatment, and the nickel (Ni) exposed on the electrode surface oxidizesto form NiO. When current is caused to flow through the electrode inthis state, the NiO reacts with an OH⁻ group of water present in thesurrounding atmosphere to form a substance comprising NiOOH, as shown bythe following scheme (1). Since NiOOH has poor wettability by gold (Au)and by the GaN related compound semiconductor, the NiOOH aggregates. Asa result, light-emitting pattern quality deteriorates with the lapse oftime and the contact resistance of the electrode increases. Thus,conventional art devices employing the proposed electrode are believedto deteriorate in optical and electrical characteristics.

NiO+OH⁻−NiOOH+e⁻  (1)

Further, since this current-diffusing electrode is thin, a pad electrodemade of Ni/Au or Au is formed thereon for bonding.

However, the conventional art device described above has insufficientadhesion between the pad electrode and the current-diffusing electrode.Hence, if the surface of the current-diffusing electrode on which a padelectrode is to be formed has been soiled, there is a problem that thefinally obtained device has problems such as the peeling of the padelectrode and a poor light-emitting pattern. In addition, even if thepad electrode has satisfactory adhesion to the current-diffusingelectrode, the light emission occurring in the shade of the bonding padcannot be directly observed, unavoidably resulting in a light emissionloss.

Further, there is still another problem as follows.

In conventional GaN related compound semiconductors, low-resistivityp-type conduction is not obtainable by mere doping with a p-typeimpurity. It has hence been proposed to impart p-type low resistance toa GaN related compound semiconductor doped with a p-type impurity byirradiating the doped semiconductor with electron beams (see JapaneseUnexamined Patent Publication No. Hei. 2-257679) or by subjecting thedoped semiconductor to thermal annealing (see Japanese Unexamined PatentPublication No. Hei. 5-183189). It has also been proposed to conduct thethermal annealing for imparting p-type low resistance simultaneouslywith alloying for forming an electrode (see Japanese Unexamined PatentPublication No. Hei. 8-51235).

However, in the method using thermal annealing described in JapaneseUnexamined Patent Publication No. Hei. 5-183189, the heat treatmentshould be conducted at a temperature not lower than 700° C. in order toobtain a saturated low resistivity. Although this kind of semiconductorhas conventionally employed aluminum as the main electrode material, useof a temperature not lower than 700° C. for electrode alloying producesproblems, such as the formation of aluminum balls resulting fromaluminum melting, an impaired surface state, increased contactresistance of the electrode, and wire bonding failure.

Consequently, the heat treatment for electrode alloying should beconducted at a relatively low temperature of from 500 to 600° C. It is,however, noted that the heat treatment for imparting p-type lowresistance does not result in a sufficiently low resistivity whenconducted at a temperature in the range of from 500 to 600° C. It hashence been necessary to conduct the heat treatment for imparting p-typelow resistance and the heat treatment for electrode alloying as separatesteps, respectively.

On the other hand, Japanese Patent Publication No. Hei. 8-51235 proposesto conduct the impartation of p-type low resistance simultaneously withelectrode alloying by performing a heat treatment at a temperature offrom 400 to 800° C. However, this method has the following problems. Theimpartation of p-type low resistance is insufficient in thelow-temperature range where electrode alloying is achievedsatisfactorily. In the high-temperature region suitable for thesufficient impartation of p-type low resistance, electrode alloyingcannot be conducted satisfactorily, resulting in increased contactresistance and poor ohmic properties.

SUMMARY OF THE INVENTION

In view of the problems described above, an object of the presentinvention is to realize a GaN related compound semiconductorlight-emitting device which has light transmission properties and ohmicproperties and retains a stable light-emitting pattern and a constantdriving voltage over a long period of time, and to realize processes forproducing the device.

Another object of the present invention is to impart p-type lowresistance to a GaN related compound semiconductor through a heattreatment so that a saturated low resistivity value can be realizedusing a lower temperature for the treatment.

Still another object of the present invention is to realize theimpartation of p-type low resistance at a lower temperature to therebysufficiently impart p-type low resistance and obtain an electrode havinglow contact resistance and satisfactory ohmic properties, even when theheat treatment for imparting p-type low resistance and that forelectrode alloying are conducted as the same step.

Still another object of the present invention is to improve the adhesionbetween a pad electrode and a current-diffusing electrode to therebyprevent the pad electrode from peeling off and, at the same time, toform a high-resistivity region under the pad so that current flows inthe current-diffusing electrode selectively through areas other thanthat under the pad to thereby diminish light emission under the pad andattain effective utilization of light emission.

The above-described problem is eliminated with the light-emitting deviceof the present invention according to a first aspect of the presentinvention. This light-emitting device has a p-type GaN related compoundsemiconductor layer having formed thereon an electrode which transmitslight to the semiconductor layer and which is a metal layer comprising acobalt (Co) alloy, palladium (Pd), or a palladium (Pd) alloy. Since theelements constituting the electrode are unsusceptible to oxidation, notonly is the electrode prevented from suffering the light-emittingpattern change with time caused by electrode oxidation to thereby give astable light-emitting pattern over a long period of time, but also theelectrode can have reduced contact resistance to thereby enable aconstant driving voltage over a long period of time. In addition, sincecobalt (Co) and palladium (Pd) each is an element having a large workfunction, satisfactory ohmic properties are obtained.

The metal layer comprising a cobalt (Co) alloy may be formed from onemember selected from the group consisting of a two-layer structurecomprising a first metal layer made of cobalt (Co) and a second metallayer made of gold (Au) formed on the first metal layer, a two-layerstructure comprising a first metal layer made of gold (Au) and a secondmetal layer made of cobalt (Co) formed on the first metal layer, and analloy layer made of cobalt (Co) and gold (Au), by alloying the onemember through a heat treatment. This metal layer is free from theproblem in electrodes made of cobalt (Co) alone that the light-emittingpattern changes with the lapse of time because of the susceptibility ofcobalt (Co) to oxidation. Specifically, the electrode formed by heatinga two-layer structure comprising a layer made of cobalt (Co) and a layermade of gold (Au) or by heating a layer of an alloy of cobalt (Co) withgold (Au) is prevented from undergoing cobalt (Co) oxidation, hasreduced contact resistance, enables a stable light-emitting pattern overa long period of time, and has excellent light transmission properties.

An electrode which has reduced contact resistance, enables a stablelight-emitting pattern over a long period of time, and has excellentlight transmission properties is also obtained from a three-layerstructure comprising a first metal layer made of cobalt (Co), a secondmetal layer made of a group II element formed on the first metal layer,and a third metal layer made of gold (Au) formed on the second metallayer, by alloying the three-layer structure through a heat treatment,or obtained from a two-layer structure comprising a first metal layermade of cobalt (Co) and a second metal layer made of an alloy ofpalladium (Pd) with platinum (Pt) formed on the first metal layer, byalloying the two-layer structure through a heat treatment. Effectiveexamples of the group II element include beryllium (Be), magnesium (Mg),calcium (Ca), strontium (Sr), barium (Ba), zinc (Zn), and cadmium (Cd).

The metal layer comprising a palladium (Pd) alloy may be formed fromeither a two-layer structure comprising a first metal layer made ofpalladium (Pd) and a second metal layer made of gold (Au) formed on thefirst metal layer, or a two-layer structure comprising a first metallayer madeiof gold (Au) and a second metal layer made of palladium (Pd)formed on the first metal layer, by alloying the two-layer structurethrough a heat treatment. Thus, an electrode is obtained which hasreduced contact resistance, enables a stable light-emitting pattern overa long period of time, and has excellent light transmission properties.

An electrode which has reduced contact resistance, enables a stablelight-emitting pattern over a long period of time, and has excellentlight transmission properties is obtained also from a layer made of analloy of palladium (Pd) with platinum (Pt) by alloying the layer througha heat treatment.

A metal layer may be formed on a p-type GaN related compoundsemiconductor layer through a heat treatment conducted at a temperaturefrom 400 to 700° C. The metal layer formed can be a satisfactorilyalloyed layer. Thus, an electrode having stable light-emittingproperties and stable electrical characteristics can be obtained.

A metal layer having reduced contact resistance can be formed through aheat treatment conducted under low-vacuum conditions. The term“low-vacuum conditions” used herein means a pressure of 10 Torr orlower.

A metal layer having reduced contact resistance can be formed through aheat treatment without reducing light-emitting pattern quality, byconducting the heat treatment in an atmosphere comprising at leastoxygen (O₂) or a gas containing oxygen (O), or by conducting the heattreatment in an inert gas atmosphere. The term “atmosphere comprisingoxygen (O₂)” as used herein include 100% oxygen (O₂). The term “gascontaining oxygen (O)” means CO, CO₂, etc. Effective examples of theinert gas contemplated by the present invention include nitrogen (N₂),helium (He), neon (Ne), argon (Ar), and krypton (Kr).

Further, the above-described problem is eliminated with the process forproducing a p-type GaN related compound semiconductor of the presentinvention according to a second aspect of the present invention. Thisprocess for producing a p-type GaN related compound semiconductorcomprises subjecting a GaN related compound semiconductor doped with ap-type impurity to a heat treatment in a gas comprising at least oxygen.

Further, the above-described problem is eliminated by the process forproducing a p-type GaN related compound semiconductor having a p-typeGaN related compound semiconductor layer and an electrode according to athird aspect of the present invention. This process for producing a GaNrelated compound semiconductor device having a p-type GaN relatedcompound semiconductor layer and an electrode comprises: forming a layerof a GaN related compound semiconductor doped with a p-type impurity;forming an electrode on the GaN related compound semiconductor layer;and subjecting the GaN related compound semiconductor layer having theelectrode formed thereon to a heat treatment in a gas comprising atleast oxygen.

Furthermore, the above-described problem is eliminated by the processfor producing GaN related compound semiconductor having a p-type GaNrelated compound semiconductor layer, an n-type GaN related compoundsemiconductor layer, and two electrodes respectively for these layersaccording to a fourth aspect of the present invention. This process forproducing a GaN related compound semiconductor device having a p-typeGaN related compound semiconductor layer, an n-type GaN related compoundsemiconductor layer, and two electrodes respectively for these layerscomprises:

forming a first electrode on the GaN related compound semiconductorlayer doped with a p-type impurity, and forming a second electrode onthe n-type GaN related compound semiconductor; and subjecting theresultant structure to a heat treatment in a gas comprising at leastoxygen.

The term “GaN related compound semiconductor” means a compound which isbased on GaN and contains one or more group III elements, e.g., In andAl, by which part of the gallium has been replaced. An example of theGaN related compound semiconductor is a four-element compoundrepresented by the general formula (Al_(x)Ga_(1−x))_(y)In_(1−y)N (O≦x≦1,0≦y≦1).

The gas comprising oxygen used in each of the processes according to thepresent invention may be at least one member selected from O₂, O₃, CO,CO₂, NO, N₂O, NO₂, and H₂O or a mixed gas comprising two or more ofthese members. The gas comprising oxygen may also be a mixed gascomprising at least one of O₂, O₃, CO, CO₂, NO, N₂O, NO₂, and H₂O andone or more inert gases, or be a mixed gas comprising a mixture of twoor more of O₂, O₃, CO, CO₂, NO, N₂O, NO₂, and H₂O and one or more inertgases. In short, the gas comprising oxygen means a gas containing oxygenatoms or a gas of molecules containing oxygen atoms.

The pressure of the atmosphere in which the heat treatment is conductedis not particularly limited as long as the GaN related compoundsemiconductor is not pyrolyzed at the temperature used for the heattreatment. In the case where O₂ gas alone is used as the gas comprisingoxygen, the gas may be introduced at a pressure higher than thedecomposition pressure for the GaN related compound semiconductor. Inthe case where a mixture of O₂ with an inert gas is used, the pressureof the whole mixed gas is regulated to a value higher than thedecomposition pressure for the GaN related compound semiconductor. Inthis case, an O₂ gas proportion not smaller than about 10⁻⁶ based on thewhole mixed gas is sufficient. In short, an extremely small amount ofoxygen suffices to the gas comprising oxygen for the reason which willbe given later. There is no particular upper limit on the amount of thegas comprising oxygen introduced from the standpoints of the impartationof p-type low resistance and electrode alloying. Any high pressure isusable as long as production is possible.

The most preferred range of the temperature for the heat treatment isfrom 500 to 600° C. As will be described later, a p-type GaN relatedcompound semiconductor having a completely saturated resistivity can beobtained at temperatures not lower than 500° C. At temperatures nothigher than 600° C., the alloying treatment of an electrode can beconducted satisfactorily.

Preferred temperature ranges are from 450 to 650° C., from 400 to 600°C., and from 400 to 700° C. The lower the temperature, the higher thep-type resistivity. The higher the temperature, the poorer the electrodeproperties and the higher the possibility for thermal deterioration ofcrystals.

The first electrode desirably comprises a metal layer which comprises acobalt (Co) alloy, palladium (Pd), or a palladium (Pd) alloy and haslight transmission properties and ohmic properties. This metal layercomprising a cobalt (Co) alloy is a layer formed from a two-layerstructure comprising a first metal layer made of cobalt (Co) and asecond metal layer made of gold (Au) formed on the first metal layer,from a two-layer structure comprising a first metal layer made of gold(Au) and a second metal layer made of cobalt (Co) formed on the firstmetal layer, or from a layer of an alloy of cobalt (Co) with gold (Au),by alloying the same through a heat treatment. Alternatively, the metallayer comprising a cobalt (Co) alloy is a layer formed from athree-layer structure comprising a first metal layer made of cobalt(Co), a second metal layer made of a group II element formed on thefirst metal layer, and a third metal layer made of gold (Au) formed onthe second metal layer, by alloying the three-layer structure through aheat treatment. The metal layer comprising a palladium (Pd) alloy is alayer formed from a two-layer structure comprising a first metal layermade of palladium (Pd) and a second metal layer made of gold (Au) formedon the first metal layer or from a two-layer structure comprising afirst metal layer made of gold (Au) and a second metal layer made ofpalladium (Pd) formed on the first metal layer, by alloying thetwo-layer structure through a heat treatment.

The first electrode can be a layer formed by alloying, through a heattreatment, a two-layer structure comprising a first metal layer made ofnickel (Ni) and a second metal layer made of gold (Au) formed thereon.

The above-described materials of the first electrode have been selectedso as to result in satisfactory properties with respect to contactresistance with p-type GaN related compound semiconductors,light-emitting pattern, property change with time, junction strength,and ohmic properties.

The second electrode desirably comprises aluminum (Al) or an aluminumalloy. These electrode materials have been selected from the standpointsof contact resistance with n-type GaN related compound semiconductorsand ohmic properties.

In the process according to the second aspect of the present invention,a gas comprising oxygen is used as the surrounding gas for the heattreatment. As a result, it has become possible to use a lowertemperature for obtaining a GaN related compound semiconductor havingp-type low resistance. As will be described later, use of temperaturesnot lower than 500° C. resulted in a saturated low value of resistivity.The resistivity began to decrease at around 400° C. At 450° C., theresistivity was about a half of that at 400° C.

In the processes according to the third and fourth aspects of thepresent invention, a saturated low resistivity suitable for practicaluse is obtained at lower temperatures as described above. Consequently,the heat treatment for imparting p-type low resistance and the heattreatment for electrode alloying can be carried out as the same step. Asa result, processes for device production can be simplified. Inaddition, since the heat treatment can be conducted at a lowtemperature, thermal deterioration of devices can be alleviated.

With respect to the fact that the heat treatment in a gas comprisingoxygen is effective in imparting low resistance at lower temperatures,the following explanation is given by the present inventors. A GaNrelated compound semiconductor cannot be made to have p-type lowresistance by merely doping the same with a p-type impurity, e.g.,magnesium. This is because the atoms of the p-type impurity are bondedto hydrogen atoms and, hence, do not function as an acceptor. It istherefore thought that upon the removal of the hydrogen atoms bonded tothe atoms of the p-type impurity, the impurity comes to function as anacceptor. When a heat treatment is conducted in a gas comprising oxygen,the separation of the impurity atoms from the hydrogen atoms is thoughtto be catalyzed by the oxygen. As a result, semiconductor devices havinga reduced resistivity are obtainable at lower temperatures.

Still further, the above-described problem is eliminated with the GaNrelated compound semiconductor device having a p-type GaN relatedcompound semiconductor according to a fifth aspect of the presentinvention. This GaN related compound semiconductor device having ap-type GaN related compound semiconductor comprises: a current-diffusingelectrode having light transmission properties which has been formed onthe p-type GaN related compound semiconductor and a pad electrode forbonding which has been formed on the current-diffusing electrode andcontains at least one metal reactive with nitrogen. The device furtherincludes a high-resistivity region on the p-type GaN related compoundsemiconductor in its part located under the pad electrode, thehigh-resistivity region having been formed through an alloying treatmentby the reaction of the metal with the p-type GaN related compoundsemiconductor.

According to this device, a current-diffusing electrode having lighttransmission properties is formed on a p-type GaN related compoundsemiconductor, and a pad electrode containing at least one metalreactive with nitrogen is formed thereon.

In an alloying treatment, the metal reactive with nitrogen which iscontained in the pad electrode reacts with the p-type GaN relatedcompound semiconductor. As a result, the adhesion between the padelectrode and the current-diffusing electrode as well as between the padelectrode and p-type GaN surface is improved and the pad electrode canbe prevented from peeling off. The reaction between the metal reactivewith nitrogen contained in the pad electrode and the GaN relatedcompound semiconductor further produces an effect that since thereaction generates nitrogen holes within part of the GaN relatedcompound semiconductor, the donor attributable to these holes in thatpart compensates for an acceptor to thereby form a high-resistivityregion in that part of the semiconductor. Consequently, current flowsfrom the pad electrode not downward but in lateral directions along thecurrent-diffusing electrode. Since the region of a pad electrodeoriginally has a large thickness and no light transmission properties,it is virtually impossible to take light out of the device through thepad electrode or to cause external light to strike on the semiconductorthrough the pad electrode. According to the present invention, only thepart where light is effectively utilizable can have an improved currentdensity and, as a result, the effective efficiency ofelectricity-to-light conversion or light-to-electricity conversion isimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a sectional view illustrating the structure of alight-emitting device according to a first embodiment of the presentinvention;

FIG. 2 is a sectional view diagrammatically illustrating the structureof an electrode formed on the surface of a p⁺ layer;

FIG. 3 is a presentation showing properties of the alloyedlight-transmitting electrodes of light-emitting devices according to thefirst embodiment of the present invention;

FIG. 4 is a presentation showing properties of the alloyedlight-transmitting electrodes of light-emitting devices according to asecond embodiment of the present invention;

FIG. 5 is a sectional view illustrating the structure of samples used inthe method for imparting p-type low resistance according a thirdembodiment of the present invention;

FIG. 6 is a graphic presentation showing changes in resistivity withchanging heat treatment temperature in samples obtained by the methodfor imparting p-type low resistance according to the third embodiment ofthe present invention;

FIG. 7 is a graphic presentation showing a change in resistivity withchanging oxygen gas pressure for heat treatment in samples obtained bythe method for imparting p-type low resistance according to the thirdembodiment of the present invention;

FIG. 8 is a graphic presentation showing a comparison between a changein resistivity with changing partial oxygen gas pressure in samplesobtained by the method for imparting p-type low resistance according tothe third embodiment of the present invention and a change inresistivity in samples obtained through heat treatment in pure oxygengas atmospheres;

FIG. 9 is a sectional view illustrating the structure of a lightemitting device produced by a process according to the third embodimentof the present invention;

FIG. 10 is a diagrammatic view illustrating the constitution of a GaNrelated compound semiconductor device according to a fourth embodimentof the present invention; and

FIG. 11 is a diagrammatic view illustrating the flow of current aroundelectrodes in the GaN related compound semiconductor device according tothe fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be explained below by reference toembodiments thereof. However, the invention should not be construed asbeing limited to the following embodiments.

1st Embodiment

FIG. 1 is a sectional view diagrammatically illustrating the structureof a light-emitting device 100 having a GaN related compoundsemiconductor formed over a sapphire substrate 1. This light-emittingdevice comprises a buffer layer 2 comprising AlN formed on the sapphiresubstrate 1 and a silicon (Si)-doped n-type CaN layer 3 (n⁺ layer)formed on the buffer layer 2. The light-emitting device furthercomprises a silicon (Si)-doped n-type Al_(0.1)Ga_(0.9)N layer 4 (nlayer) having a thickness of 0.5 μm formed on the n⁺ layer 3, anIn_(0.2)Ga_(0.8)N layer 5 (active layer) having a thickness of 0.4 μmformed on the n layer 4, and a magnesium (Mg)-doped p-typeAl_(0.1)Ga_(0.9)N layer 6 (p layer) formed on the active layer 5. Ap-type GaN layer 7 (p⁺ layer) heavily doped with magnesium (Mg) has beenformed on the p layer 6. A light-transmitting electrode 8A has beenformed on the p⁺ layer 7 by metal vapor deposition, while an electrode8B has been formed on the n⁺ layer 3. The light-transmitting electrode8A is constituted of cobalt (Co) bonding to the p⁺ layer 7 and of ametallic element, e.g., gold (Au), bonding to the cobalt (Co) (themetallic element will be described later). The electrode 8B isconstituted of aluminum (Al) or an aluminum alloy.

A process for producing the light-transmitting electrode 8A of thislight-emitting device 100 is explained next.

The layers ranging from the buffer layer 2 to the p⁺ layer 7 are formedby metal-organic vapor-phase epitaxy (MOVPE). The gases which can beused are ammonia (NH₃), carrier gases (H₂, N₂), trimethylgallium(Ga(CH₃)₃) (hereinafter referred to as “TMG”), trimethylaluminum(Al(CH₃)₃) (hereinafter referred to as “TMA”), silane (SiH₄),cyclopentadienylmagnesium (Mg(C₅H₅)₂) (hereinafter referred to as“Cp₂Mg”), and trimethylindium (In(CH₃)₃) (hereinafter referred to as“TMI”). And a mask layer (SiO₂ or the like) is formed on the p⁺ layer 7,and the predetermined area of the mask layer is removed. Those parts ofthe p⁺ layer 7, p layer 6, active layer 5, and n layer 4 which areuncovered by the resultant mask are removed by reactive ion etching witha gas containing chlorine to expose a surface of the n⁺ layer. The maskis then removed.

Subsequently, the light-transmitting electrode 8A is formed byconducting the following procedure.

A photoresist 9 is evenly applied to the surface. That part of thephotoresist 9 which corresponds to the area where the electrode is to beformed on the p⁺ layer 7 is removed by photolithography to form a windowpart 9A as shown in FIG. 2.

Using a vapor deposition apparatus, cobalt (Co) is deposited in athickness of 40 Å on the exposed p⁺ layer 7 under a high vacuum on theorder of 10⁻⁶ Torr or less to form a first metal layer 81 as shown inFIG. 2.

Gold (Au) is then deposited on the first metal layer 81 in a thicknessof 60 Å to form a second metal layer 82 as shown in FIG. 2.

Subsequently, the sample is taken out of the vapor deposition apparatus.The cobalt and gold deposited on the photoresist 9 are removed by thelift-off method to form an electrode 8A which transmits light to the p⁺layer 7.

In the case where a bonding electrode pad 20 is to be formed on part ofthe light-transmitting electrode 8A, a photoresist is applied evenly,and that part of the photoresist which corresponds to the pad formationpart is removed to form a window. Subsequently, a film of an alloy ofcobalt (Co) or nickel (Ni) with gold (Au), aluminum (Al) or both isformed by vapor deposition in a thickness of about 1.5 μm. The filmalloy of cobalt or nickel with gold, aluminum, or both which has beenvapor-deposited on the photoresist is removed by the lift-off method asmentioned above to thereby form an electrode pad 20.

Thereafter, the atmosphere surrounding the sample is evacuated with avacuum pump, and a mixed gas of N₂ and O₂ (1%) is introduced into thedeposition apparatus to adjust the internal pressure to atmosphericpressure. The temperature of this atmosphere surrounding the sample iselevated to about 550° C. to heat the sample for about 3 minutes. Thus,the first metal layer 81 and the second metal layer 82 are alloyed.

This heat treatment can be conducted under the following conditions. Asurrounding gas containing one or more of N₂, He, O₂, Ne, Ar, and Kr isutilizable. Any pressure ranging from vacuum to pressures higher thanatmospheric pressure can be used. The partial pressure of N₂, He, O₂,Ne, Ar, or Kr in the surrounding gas is from 0.01 to 1 atm. The heatingmay be conducted with the surrounding gas enclosed in the apparatus orwhile circulating the same through the apparatus.

As a result of the heat treatment after the deposition of cobalt (Co)and gold (Au), part of the gold (Au) constituting the second metal layer82 formed on the first metal layer 81 made of cobalt (Co) is diffusedthrough the first metal layer 81 on the p⁺ layer 7 to thereby form agood contact with GaN contained in the p⁺ layer 7.

When a current of 20 mA was caused to flow through the thus-formedlight-transmitting electrode 8A, a driving voltage of 3.6 V wasobtained. It was thus ascertained that the contact resistance wassufficiently low. The surface of the p₊ layer 7 was evenly covered withthe thus-formed light-transmitting electrode 8A, which had asatisfactory surface state.

Since the light-transmitting electrode 8A is formed from a two-layerstructure comprising the first metal layer 81 made of cobalt (Co) andthe second metal layer 82 formed thereon, the cobalt (Co) is inhibitedfrom oxidizing. As a result, the change of light-emitting pattern,decrease of light transmission properties, and increase of contactresistance all caused by cobalt (Co) oxidation can be prevented. Inaddition, since the light-transmitting electrode 8A is made of an alloycontaining cobalt (Co), which has a large work function, satisfactoryohmic properties are obtained. This electrode 8A was tested by exposingthe same to a high-temperature and high-humidity atmosphere for aprolonged period of time. As a result, the electrode was capable ofstably maintaining the initial light-emitting pattern and drivingvoltage even after 1,000-hour exposure.

Besides the conditions used above for alloying the first metal layer 81made of cobalt (Co) and the second metal layer 82 made of gold (Au), thefollowing two sets of conditions were also used for this embodiment. Oneset of conditions was that the atmosphere surrounding the sample wasevacuated with a vacuum pump to form a low-vacuum state and thetemperature of this atmosphere surrounding the sample was elevated toabout 550° C. to heat the sample for about 3 minutes to thereby alloythe first and second metal layers 81 and 82. The other set of conditionswas that the atmosphere surrounding the sample was evacuated to vacuum,subsequently N₂ was introduced at a rate of 3 liter/min to adjust theinternal pressure to atmospheric pressure, and then the temperature ofthis atmosphere surrounding the sample was elevated to about 550° C. toheat the sample for about 3 minutes to thereby alloy the first andsecond metal layers 81 and 82. The driving voltage of each deviceobtained was measured. The results obtained are shown in FIG. 3 underCase No. 1.

The three sets of atmospheric conditions described above were used forthe alloying of: an electrode precursor comprising a first metal layer81 made of gold (Au) and a second metal layer 82 made of cobalt (Co)(Case No. 2); an electrode precursor having only of a first metal layer81 comprising an alloy of cobalt (Co) with gold (Au) (Case No. 3); athree-layer precursor for light-transmitting electrode which wasconstituted by a first metal layer 81 made of cobalt (Co), a secondmetal layer 82 made of magnesium (Mg), and a third metal layer made ofgold (Au) formed on the second metal layer (Case No. 4); and anelectrode precursor comprising of a first metal layer 81 made of cobalt(Co) and a second metal layer 82 made of an alloy of palladium (Pd) withplatinum (Pt) (Case No. 5). The driving voltage of each device obtainedwas measured. The results obtained are shown in FIG. 3.

The evaluation results given in FIG. 3 are based on the driving voltagemeasured when a current of 20 mA was caused to flow through thelight-transmitting electrode 8A. In FIG. 3, ∘ indicates that the drivingvoltage was lower than 4 V, and x indicates that the driving voltage wasnot lower than 5 V. In FIG. 3, the numeral in the parentheses for eachmetal layer indicates film thickness (Å).

All the device samples described above were subjected to a 1,000-hourcontinuous driving test in a high-temperature high-humidity atmosphere.The device samples indicated by ∘ each had the same driving voltage andlight-emitting pattern as the initial ones even after the 1,000-hourdriving test, and retained optically and electrically stable propertiesover a long period of time.

In Case No. 1 shown in FIG. 3, a device having a driving voltage notlower than 5 V was measured at a current of 20 mA. Hence, increasedcontact resistance was obtained when alloying was conducted in N₂(alone) in the absence of O₂. Through alloying under low-vacuumconditions, a device having a driving voltage lower than 4 V, and hencereduced contact resistance, was obtained. In the case where alight-transmitting electrode 8A is formed from a two-layer structurecomprising a first metal layer 81 made of cobalt (Co) and a second metallayer 82 made of gold (Au) as in Case No. 1, a light-emitting patternwhich is stable over a long period of time and a low driving voltage isobtained by alloying the two-layer structure either in the atmospherecontaining O² or under the low-vacuum conditions.

As in Case No. 2 shown in FIG. 3, a first metal layer 81 made of gold(Au) having a thickness of 40 Å may be formed before a second metallayer 82 made of cobalt (Co) is formed thereon with a thickness of 60 Å.A light-emitting pattern which is stable over a long period of time anda low driving voltage are obtained in Case No. 2 by conducting alloyingeither in the atmosphere containing O₂ or under the low-vacuumconditions, as in Case No. 1.

As in Case No. 3 shown in FIG. 3, a first metal layer 81 may be formedby simultaneously vapor-depositing gold (Au) and cobalt (Co) with athickness of 100 Å. A light-emitting pattern which is stable over a longperiod of time and a low driving voltage are obtained in Case No. 3 byconducting alloying either in the atmosphere containing O₂ or under thelow-vacuum conditions, as in Cases Nos. 1 and 2.

As in Case No. 4 shown in FIG. 3, a light-transmitting electrode 8Ahaving a three-layer structure may be formed by forming an electrodeprecursor comprising a first metal layer 81 made of cobalt (Co) having athickness of 20 Å, a second metal layer 82 formed thereon which is madeof magnesium (Mg) having a thickness of 20 Å, and a 60 Å-thick layer ofgold (Au) formed on the second metal layer 82. In Case No. 4, any of theatmosphere containing O₂ the low-vacuum conditions, and the N₂atmosphere can be used for obtaining a light-emitting pattern which isstable over a long period of time and a low driving voltage.

As in Case No. 5 shown in FIG. 3, palladium (Pd) and platinum (Pt) maybe simultaneously vapor-deposited as a second metal layer 82 with athickness of 80 Å on a first metal layer 81 made of cobalt (Co) having athickness of 40 Å. A light-emitting pattern which is stable over a longperiod of time and a low driving voltage can be obtained in Case No. 5by conducting alloying either under the low-vacuum conditions or in theN₂ atmosphere.

As described above, the light-transmitting electrode 8A may be formedfrom a two-layer structure comprising a first metal layer 81 made ofcobalt (Co) and a second metal layer 82 formed thereon, or from atwo-layer structure comprising a first metal layer 81 made of gold (Au)and a second metal layer 82 made of cobalt (Co) formed thereon, or froma single-layer structure comprising a first metal layer 81 made of agold (Au)-cobalt (Co) alloy.

Although magnesium (Mg) was used as the material of a constituent metallayer in Case No. 4 in the embodiment described above, other group IIelements may be used, such as beryllium (Be), calcium (Ca), strontium(Sr), barium (Ba), zinc (Zn), and cadmium (Cd).

2nd Embodiment

In contrast to the first embodiment described above, in which cobalt(Co) was used as the first metal layer 81 or second metal layer 82, thisembodiment is characterized by employing a light-transmitting electrode8A which is made of palladium (Pd) alone or a palladium (Pd) alloy, andcontains no cobalt (Co).

The semiconductor devices used have the same constitution as in thefirst embodiment, except the composition of the light-transmittingelectrode 8A. FIG. 4 shows the relationship between the composition ofeach of the precursors for the light-transmitting electrode 8A and thedriving voltage as measured when a current of 20 mA was caused to flowthrough the light-transmitting electrode 8A after the precursor wasalloyed under each of the same sets of conditions as those used for thefirst embodiment. The ratings used in FIG. 4 have the followingmeanings: ∘ indicates that the driving voltage was lower than 4 V; Δindicates that the driving voltage was 4 V or higher but below 5 V; andx indicates that the driving voltage was not lower than 5 V. The devicesamples indicated by ∘ or Δ each had the same driving voltage andlight-emitting pattern as the initial ones even after 1,000-hour drivingtest, and retained optically stable properties over a long period oftime.

As in Case No. 1, a light-emitting pattern which is stable over a longperiod of time and a driving voltage as low as below 4 V were obtainedby forming an electrode precursor comprising a first metal layer 81 madeof 40 Å-thick palladium (Pd) formed on the p⁺ layer 7 and a second metallayer 82 made of 60 Å-thick gold (Au) formed on the first metal layer81, and then alloying the precursor under any of the three sets ofconditions. Thus, the same effects as in the first embodiment could beobtained. In addition, since the light-transmitting electrode 8A wasmade of an alloy of palladium (Pd), which has a large work function,satisfactory ohmic properties were obtained as in the first embodiment.

As in Case No. 2, a light-emitting pattern which was stable over a longperiod of time and a low driving voltage were obtained by forming anelectrode precursor comprising a first metal layer 81 made of 40 Å-thickgold (Au) formed on the p⁺ layer 7 and a second metal layer 82 made of60 Å-thick palladium (Pd) formed on the first metal layer 81, and thenalloying the precursor under any of the three sets of conditions. Thus,the same effects as in Case No. 1 were obtained.

In Cases Nos. 1 and 2, two-layer structures were used for forminglight-transmitting electrodes 8A. In contrast thereto, a 100 Å-thicksingle-layer structure was formed by simultaneously vapor-depositingpalladium (Pd) and platinum (Pt) and was alloyed under low-vacuumconditions to form a light-transmitting electrode 8A, as in Case No. 3,whereby a light-emitting pattern which was stable over a long period oftime and a low driving voltage were obtained.

Furthermore, as in Case No. 4, a light-transmitting electrode 8A wasformed by forming a 100 Å-thick single-layer structure made of palladium(Pd) and alloying the structure under low-vacuum conditions, whereby alight-emitting pattern which was stable over a long period of time and alow driving voltage were obtained. A driving voltage of from 4 to 5 Vwas obtained when the single-layer structure was alloyed in an N₂atmosphere.

As described above, by forming a light-transmitting electrode BA made ofan alloy of palladium (Pd) with gold (Au) or platinum (Pt) or made ofpalladium (Pd) alone, a light-emitting pattern which was stable over along period of time and a low driving voltage were obtained and the sameeffects as in the first embodiment could be obtained.

Although the temperature of the atmospheres used for alloying forproducing the embodiments described above was regulated to about 550°C., usable alloying temperatures are not limited thereto. The heattreatment is desirably conducted at a temperature in the range of from400 to 700° C. This is because heat treatments conducted at temperatureslower than 400° C. result in electrodes not showing ohmic properties,while heat treatments conducted at temperatures higher than 700° C.result in electrodes having increased contact resistance and an impairedsurface morphology.

The light-emitting devices 100 shown above as embodiments of theinvention each had a structure containing an active layer 5 consistingof a single layer of In_(0.2)Ga_(0.8)N. However, the light-emittingdevice of the invention may have a light-emitting layer which is made ofa mixed crystal comprising four or three elements in any proportion,e.g., AlInGaN, or has a multi-quantum well structure consisting, e.g.,of In_(0.2)Ga_(0.8)N/GaN or a single-quantum well structure.

In producing the embodiments described above, an atmosphere containing1% O₂ was used as an oxygen-containing atmosphere. However, a 100% O₂atmosphere or an atmosphere containing a gas such as CO or CO₂ may beused.

The total thickness of the light-transmitting electrode 8A, includingthe first metal layer 81 and the second metal layer 82, is preferablynot larger than 200 Å from the standpoint of obtaining lighttransmission properties. It is more preferably in the range of from 15to 200 Å from the standpoints of adhesion and light transmissionproperties.

As shown above, the present invention brings about the followingeffects. By forming a metal layer comprising a cobalt (Co) alloy,palladium (Pd), or a palladium (Pd) alloy as a light-transmittingelectrode on a surface of a semiconductor comprising a p-type GaNrelated compound, not only can the electrode be inhibited from oxidizingto thereby prevent the electrode from suffering a decrease in lighttransmission properties, but the electrode can also have reduced contactresistance to thereby enable a light-emitting pattern which is stableover a long period of time and a low driving voltage.

3rd Embodiment

The present invention will be explained below by reference to FIGS. 5 to9.

Many samples having the structure shown in FIG. 5 were prepared. Eachsample was constituted by a sapphire substrate 1 and, formed thereon inthis order, an AlN buffer layer 2 having a thickness of 50 nm, an n-GaNlayer 103 made of a silicon (Si)-doped GaN having a thickness of about4.0 μm, an electron concentration of 2×10¹⁸/cm³, and a siliconconcentration of 4×10¹⁸/cm³, and a p-GaN layer 104 having a magnesium(Mg) concentration of 5×10¹⁹/cm³.

These samples were produced by MOVPE, like the aforementionedlight-emitting devices 100.

First, a single-crystal sapphire substrate 1 having, as the mainsurface, a surface which had been cleaned by organic washing and heattreatment was mounted on a susceptor placed in the reaction chamber ofan MOVPE apparatus. The sapphire substrate 1 was baked at 1,100° C.while passing Hz through the reaction chamber at a rate of 2 liter/minfor about 30 minutes at ordinary pressure.

After the temperature of the substrate 1 was lowered to 400° C., H₂,NH₃, and TMA were fed for about 1.5 minutes at rates of 20 liter/min, 10liter/min, and 1.8×10⁻⁵ mol/min, respectively, to form an AlN bufferlayer 2 in a thickness of about 50 nm.

Subsequently, while keeping the temperature of the sapphire substrate 1at 1,150° C., H₂, NH₃, TMG, and silane diluted with H₂ gas to 0.86 ppmwere fed for 40 minutes at rates of 20 liter/min, 10 liter/min, 1.7×10⁻⁴mol/min, and 20×10⁻⁸ mol/min, respectively, to form an n-GaN layer 103having a thickness of about 4.0 μm, an electron concentration of2×10¹⁸/cm³, and a silicon concentration of 4×10¹⁸/cm³.

Thereafter, while keeping the temperature of the sapphire substrate 1 at1,100° C., either N₂ or H₂, NH₃, TMG, and CP₂Mg were fed for 40 minutesat rates of 10 liter/min, 10 liter/min, 1.7×10⁻⁴ mol/min, and 2×10⁻⁵mol/min, respectively, to form a p-GaN layer 104 having a thickness ofabout 4.0 μm and a magnesium (Mg) concentration of 5×10¹⁹/cm³.

Many samples thus prepared were subjected to a 20-minute heat treatmentat various temperatures in a 1-atm oxygen gas atmosphere (only of O₂).Needle electrodes were set up on each of the thus-treated p-GaN layers104 to measure the current which flowed upon application of a voltage of8 V, and the relationship between this current value and the heattreatment temperature used was determined. On the other hand, for thepurpose of comparison, semiconductor samples were subjected to the sameheat treatment as the above, except that 1-atm nitrogen gas (only of N₂)was used as the atmosphere for the heat treatment as in conventionalprocesses, and the relationship between current value and heat treatmenttemperature was determined in the same manner. The values of resistivitywere calculated, and the relationships between heat treatmenttemperature and resistivity are shown in FIG. 6.

The following features can be understood from FIG. 6. 1) Both the heattreatment in oxygen atmosphere and the heat treatment in nitrogenatmosphere result in a decrease in resistivity [(resistivity before heattreatment)/(resistivity after heat treatment)] of 10⁴. Namely, there isno difference in the saturated resistivity value between the two kindsof heat treatments. 2) The saturated low resistivity value is obtainedby a treatment at lower temperatures in the oxygen atmosphere than inthe nitrogen atmosphere. 3) The heat treatment in oxygen atmosphereresults in a more abrupt change in resistivity with changing heattreatment temperature than the heat treatment in nitrogen atmosphere. 4)The heat treatment in oxygen atmosphere at 500° C. results in asaturated low resistivity value, whereas the heat treatment in nitrogenatmosphere at 500° C. results in a resistivity change as small as about10. Namely, the resistivity resulting from the heat treatment in oxygenatmosphere at 500° C. is lower by 10³ than that resulting from the heattreatment in nitrogen atmosphere at 500° C. 5) At 400° C., both the heattreatment in an oxygen atmosphere and that in a nitrogen atmosphereresult in almost no decrease in resistivity. At temperatures higher than400° C., the heat treatments are effective in reducing resistivity.

In summary, in the oxygen atmosphere, heating at temperatures not lowerthan 400° C. is effective in lowering resistivity. The heat treatment isdesirably conducted at a temperature not lower than 500° C., becausethis treatment provides the completely saturated low resistivity value.

The relationship between the pressure of oxygen gas and resistivity wasthen determined. At a temperature of 800° C., semiconductor samples wereheat-treated at various pressures of oxygen gas. Needle electrodes wereset up on each of the thus-treated p-GaN layers 104 to measure thecurrent which flowed upon application of a voltage of 8 V, and therelationship between this current value and oxygen gas pressure wasdetermined. The results obtained are shown in FIG. 7.

The following characteristics are understood from these results. 1)Resistivity drops abruptly in the oxygen gas pressure range of aboutfrom 3 to 30 Pa. 2) The heat treatment at oxygen gas pressures not lowerthan about 100 Pa results in a saturated low resistivity value.

It is understood from the above that oxygen contributes to the effectivereduction of resistivity. Oxygen gas pressures of at least 3 Pa areeffective in reducing resistivity. The oxygen gas atmosphere preferablyhas an oxygen pressure of 30 Pa or higher, more preferably 100 Pa.

Subsequently, semiconductor samples were heat-treated at a temperatureof 600° C. in mixed gas atmospheres (1 atm) containing oxygen gas andnitrogen gas to determine the change of resistivity with changingpartial oxygen gas pressure in the same manner as the above. For thepurpose of comparison, the change of resistivity with changing pressurein heat treatment in oxygen gas alone was determined. The resultsobtained are shown in FIG. 8. The results show that at partial oxygengas pressures not lower than about 10 Pa, low resistivities areobtained. The results further show that the resistivity is saturated atpressures not lower than 30 Pa, ideally not lower than 100 Pa. To sumup, in the case of using a mixed gas containing oxygen gas and one ormore other gases, the partial pressure of oxygen gas effective inreducing resistivity is 10 Pa or higher, preferably 30 Pa or higher,more preferably 100 Pa or higher.

A layer of a magnesium-doped GaN related compound semiconductorrepresented by (Al_(x)Ga_(1−x))_(y)In_(1−y)N (O≦x,y≦1) gave the sameresults with respect to all the properties described above. It isthought that oxygen serves to remove the hydrogen atoms bonded tomagnesium and to thereby activate the magnesium atoms. Consequently,besides pure oxygen gas, any gas containing oxygen (O) atoms capable ofbonding to hydrogen atoms bonded to magnesium, e.g., a mixed gascontaining oxygen and an inert gas, may be used to produce the sameeffect.

An explanation is given below on a process for producing alight-emitting device 100 using the above-described method for impartingp-type low resistance by reference to FIG. 9. FIG. 9 is a sectional viewdiagrammatically illustrating the structure of a light-emitting device100 having a GaN related compound semiconductor formed over a sapphiresubstrate 1. This light-emitting device 100 has the substantially samestructure as the aforementioned embodiments. However, in thisembodiment, a clad layer 114 made of silicon (Si)-doped n-type GaN isformed on the high-carrier-concentration n⁺ layer 3.

Further, on the clad layer 114 has been formed a light emitting layer115 having a multi-quantum well structure (MQW) comprising barrierlayers 151 made of GaN each having a thickness of 35 Å and well layers152 made of In_(0.20)Ga_(0.80)N each having a thickness of 35 Å. Thenumber of the barrier layers 151 is six, while the number of the welllayers 152 is five. On the light-emitting layer 115 has been formed aclad layer 116 made of p-type Al_(0.15)Ga_(0.85)N. A contact layer 117made of p-type GaN is formed on the clad layer 116.

A process for producing this light-emitting device 100 is explained nexttogether with the steps which are not explained in the first embodiment.

The light-emitting device 100 was produced by MOVPE. The gases used wereammonia (NH₃), carrier gases (H₂, N₂), TMG, TMA, TMI, silane, and CP₂Mg.

First, a single-crystal sapphire substrate 1 having, as the mainsurface, a surface which had been cleaned by organic washing and heattreatment was mounted on a susceptor placed in the reaction chamber ofan MOVPE apparatus. The sapphire substrate 1 was baked at 1,100° C.while passing H₂ through the reaction chamber at a rate of 2 liter/minfor about 30 minutes at ordinary pressure.

After the temperature of the substrate 1 was lowered to 400° C., H₂,NH₃, and TMA were fed for about 1 minute at rates of 20 liter/min, 10liter/min, and 1.8×10⁻⁵ mol/min, respectively, to form an AlN bufferlayer 2 in a thickness of about 25 nm.

Subsequently, while keeping the temperature of the sapphire substrate 1at 1,150° C., H₂, NH₃, TMG, and silane diluted with H₂ gas to 0.86 ppmwere fed for 40 minutes at rates of 20 liter/min, 10 liter/min, 1.7×10⁻⁴mol/min, and 20×10⁻⁸ mol/min, respectively, to form ahigh-carrier-concentration n⁺ layer 3 made of GaN and having a thicknessof about 4.0 μm, an electron concentration of 2×10¹⁸/cm³, and a siliconconcentration of 4×10¹⁸/cm³.

Thereafter, while keeping the temperature of the sapphire substrate 1 at1,150° C., either N₂ or H₂, NH₃, TMG, TMA, and silane diluted with H₂gas to 0.86 ppm were fed for 60 minutes at rates of 10 liter/min, 10liter/min, 1.12×10⁻⁴ mol/min, 0.47×10⁻⁴ mol/min, and 5×10⁻⁹ mol/min,respectively, to form a clad layer 114 made of GaN and having athickness of about 0.5 μm, an electron concentration of 1×10¹⁸/cm³, anda silicon concentration of 2×10¹⁸/cm³.

Subsequent to the formation of the clad layer 114, either N₂ or H₂, NH₃,and TMG were fed for 1 minute at rates of 20 liter/min, 10 liter/min,and 2.0×10⁻⁴ mol/min, respectively, to form a barrier layer 151 made ofGaN and having a thickness of about 35 Å. Subsequently, TMG and TMI werefed for 1 minute at rates of 7.2×10⁻⁵ mol/min and 0.19×10⁻⁴ mol/min,respectively, while feeding either N₂ or H₂ and NH₃ at constant rates tothereby form a well layer 152 made of In_(0.20)Ga_(0.80)N and having athickness of about 35 Å. Under the same conditions as the above, fivebarrier layers 151 in total were formed alternately with five welllayers 152 in total. Furthermore, a barrier layer 151 made of GaN wasformed thereon. Thus, a light-emitting layer 115 of the 5-cycle MQWstructure was formed.

Thereafter, while keeping the temperature of the sapphire substrate 1 at1,100° C., either N₂ or H₂, NH₃, TMG, TMA, and CP₂Mg were fed for 3minutes at rates of 10 liter/min, 10 liter/min, 1.0×10⁻⁴ mol/min,1.0×10⁻⁴ mol/min, and 2×10⁻⁵ mol/min, respectively, to form a clad layer116 made of magnesium (Mg)-doped p-type Al_(0.15)Ga_(0.85)N and having athickness of about 50 nm and a magnesium (Mg) concentration of5×10¹⁹/cm³.

Subsequently, while keeping the temperature of the sapphire substrate 1at 1,100° C., either N₂ or H₂, NH₃, TMG, and CP₂Mg were fed for 30seconds at rates of 20 liter/min, 10 liter/min, 1.12×10⁻⁴ mol/min, and2×10⁻⁵ mol/min, respectively, to form a contact layer 117 made ofmagnesium (Mg)-doped p-type GaN and having a thickness of about 100 nmand a magnesium (Mg) concentration of 5×10¹⁹/cm³.

In this embodiment, a window is formed in a predetermined region of thephotoresist by photolithography on the contact layer 117 through theaforementioned steps in the first embodiment. Under a high vacuum on theorder of 10⁻⁶ Torr or below, vanadium (V) and aluminum (Al) arevapor-deposited in thickness of 200 Å and 1.8 μm, respectively. Thephotoresist and the SiO₂ mask are then removed.

Subsequently, a photoresist 9 is evenly applied to the surface. Thatpart of the photoresist 9 which corresponds to the area where theelectrode is to be formed on the contact layer 117 is removed byphotolithography to form a window part 9A as shown in FIG. 2.

Using a vapor deposition apparatus, a first metal layer 81 made ofcobalt (Co) is formed in a thickness of 15 Å on the exposed contactlayer 117 under a high vacuum on the order of 10⁻⁶ Torr or below, and asecond metal layer 82 made of gold (Au) is then formed in a thickness of60 Å on the first metal layer 81.

Subsequently, the electrode 8A and electrode pad 20 are formed by thesame processes as in the first embodiment.

Thereafter, the atmosphere surrounding the sample is evacuated with avacuum pump, and O₂ gas is introduced into the deposition apparatus toadjust the internal pressure to 100 Pa. The temperature of thisatmosphere surrounding the sample is elevated to about 550° C. to heatthe sample for about 3 minutes. Thus, p-type low resistance is impartedto the contact layer 117 and clad layer 116 and, at the same time, thealloying of the contact layer 117, first metal layer 81, and secondmetal layer 82 and the alloying of the electrode 8B and n⁺ layer 3 areconducted.

As a result of this heat treatment, the resistivity of the contact layer117 and that of the clad layer 116 became 1 Ωcm and 0.71 Ωcm,respectively. The most preferred range of the temperature for this heattreatment is from 500 to 600° C. As long as the heat treatment isconducted at a temperature in this range, the p-type layer reaches asufficiently low saturated resistivity value and the electrodes 8A and8B are alloyed most satisfactorily. As a result, not only can thecontact resistance of electrodes or the sheet resistivity of thecurrent-diffusing electrode be reduced and ohmic properties improved,but also the light-transmitting electrode 8A is prevented fromoxidizing, whereby the finally obtained light-emitting device can befree from an uneven light-emitting pattern and undergo no change inlight-emitting pattern with the lapse of time. The heat treatment can beconducted at a temperature of from 450 to 650° C., and can be conductedeven in the range of from 400 to 700° C. in some cases. Heat treatmentwas further conducted in an atmosphere containing a mixture of N₂ gasand 1% O₂ gas and having a partial O₂ gas pressure of 100 Pa. As aresult, the same effects as the above were obtained. All of the gasesenumerated above which are used as the surrounding gas for heattreatment with regard to the impartation of p-type low resistance arealso effective in the alloying of the electrodes 8A and 8B described inthe first embodiment. Consequently, besides pure oxygen gas, a mixed gascan be utilized which contains O₂ and at least one of N₂, He, Ne, Ar,and Kr. Any pressure and any partial O₂ pressure within theaforementioned optimal ranges for the impartation of p-type lowresistance are utilizable.

As a result of the heat treatment after the deposition of cobalt (Co)and gold (Au), part of the gold (Au) constituting the second metal layer82 formed on the first metal layer 81 made of cobalt (Co) is diffusedthrough the first metal layer 81 into the contact layer 117 to therebyform a goof contact with GaN contained in the contact layer 117.

It was ascertained that the light-emitting device 100 in this embodimentdesignates sufficiently low contact resistance and the stability withrespect to the 1,000 hour continuous driving test just like theaforementioned embodiments.

Although magnesium (Mg) was used in a metal layer described above, itmay be replaced by another group II element such as, e.g., beryllium(Be), calcium (Ca), strontium (Sr), barium (Ba), zinc (Zn), or cadmium(Cd).

Further, it is possible to apply other structures or other elements forthe light-transmitting electrode 8A, the first metal layer 81, thesecond metal layer 82, the light-emitting layer 115 as described in theaforementioned embodiments.

4th Embodiment

The fourth embodiment of the present invention will be explained belowby reference to FIGS. 10 and 11.

FIG. 10 is a sectional view diagrammatically illustrating theconstitution of a light-emitting device 100 having a GaN relatedcompound semiconductor formed over a sapphire substrate 1. Thelight-emitting device 100 was produced by MOVPE as in the aforementionedembodiments.

This light-emitting device 100 has the substantially same structure asthe third embodiment. However, on a part of the electrode 8A, a padelectrode 20 has been formed comprising a first metal layer 201 about300 Å thick made of vanadium (V) and a second metal layer 202 having atwo-layer structure comprising a cobalt layer about 1,000 Å thick and agold layer about 1.5 μm thick. The process for forming this electrodepad is as follows.

A vanadium film about 300 Å thick is deposited on a part of thiselectrode 8A to form a first metal layer 201. On the first metal layer201 are successively deposited a cobalt film about 1,000 Å thick and agold film about 1.5 μm-thick to form a second metal layer 202. Thus, theelectrode pad 20 is formed.

After the formation of the electrodes 8A, 8B, and pad 20, p-type lowresistance is imparted to the contact layer 117 and clad layer 116 and,at the same time, the alloying of the contact layer 117, metal layers 81and 82, first metal layer 201, and second metal layer 202 was conductedsimultaneously with the alloying of the electrode 8B and n⁺ layer 3 bythe same process as described in the previous embodiments.

As shown in the embodiment described above, the first metal layer 201 ofthe electrode pad 20, which layer is bonded to the electrode 8A, isconstituted of vanadium, which is reactive with nitrogen. Consequently,in an alloying treatment, the vanadium reacts with GaN of the contactlayer 117 to improve the adhesion between the electrode pad 20 and theelectrode 8A, whereby the electrode pad 20 can be prevented from peelingoff.

Furthermore, as a result of the reaction of vanadium with GaN of thecontact layer 117, nitrogen holes generate within the contact layer 117.Since the donor attributable to these holes compensates for an acceptorto result in a reduced hole concentration, a high-resistivity region 171is formed under the electrode pad 20 around the junction of the contactlayer 117 with the electrode 8A, as shown in FIG. 11. Due to theformation of this high-resistivity region 171, current flows from theelectrode pad 20 not downward but in lateral directions along theelectrode 8A. The electrode pad 20 is a thick part which has no lighttransmission properties and through which light cannot generally pass.By thus causing the current which has passed through the electrode pad20 to flow along the electrode 8A, through which light can pass, theelectrode 8A has an increased current density and an improved luminancecan be obtained.

In the embodiment described above, vanadium was used as the material ofthe first metal layer 201. However, use of a chromium (Cr) film about300 Å thick as the first metal layer 201 was also found to be effective,like the vanadium film, in obtaining tenacious adhesion and in causingcurrent to flow from the electrode pad 20 not downward but selectivelyalong the electrode 8A.

Although chromium or vanadium was used for the first metal layer 201 inthe above embodiment, the layer 201 may be constituted of at least oneof chromium, vanadium, titanium (Ti), niobium (Nb), tantalum (Ta), andzirconium (Zr). Although cobalt and gold were used for the second metallayer 202, this layer may be constituted of at least one of cobalt,nickel, aluminum, and gold. It is also possible to use two or more ofthese materials to form an electrode pad 20 of a single-layer structureby simultaneous vapor deposition.

The electrode 8A may further contain palladium or a palladium alloy. Aslong as these materials are used, the electrode 8A may have asingle-layer structure or a multilayer structure comprising three ormore layers as the aforementioned embodiments.

In the embodiment described above, the heating for alloying wasconducted at a temperature of 550° C. However, temperatures in the rangeof from 400 to 700° C. are usable.

In the embodiment.described above, the heat treatment was conducted inan O₂ gas atmosphere (as described in the first and third embodiments).However, the atmosphere for heat treatment may consist of at least onemember selected from O₂, O₃, CO, CO₂, NO, N₂O, NO₂, and H₂O or a mixedgas containing two or more of these. The atmosphere for heat treatmentmay also be a mixed gas containing at least one of O₂, O₃, CO, CO₂, NO,N₂O, NO₂, and H₂O and one or more inert gases, or be a mixed gascontaining a mixture of two or more of O₂, O₃, CO, CO₂, NO, N₂O, NO₂,and H₂O and one or more inert gases. In short, the atmosphere for heattreatment may be any gas containing either oxygen atoms or moleculeshaving oxygen atoms. In the heat treatment, the hydrogen atoms bonded toatoms of a p-type impurity in the contact layer 117 are heated in a gascomprising oxygen and are thereby separated from the p-type impurityatoms. As a result, the contact layer 117 can have lower resistance.

In the embodiment described above, alloying was conducted in an O₂ gasatmosphere having a pressure of 3 Pa. However, the pressure of theatmosphere for heat treatment is not particularly limited as long as theGaN related compound semiconductor is not pyrolyzed at the temperatureused for the heat treatment. In the case where O₂ gas alone is used as agas comprising oxygen, the gas may be introduced at a pressure higherthan the decomposition pressure for the GaN related compoundsemiconductor. In the case where a mixture of O₂ with an inert gas isused, the pressure of the whole mixed gas is regulated to a value higherthan the decomposition pressure for the CaN related compoundsemiconductor. In this case, an O₂ gas proportion not smaller than about10⁻⁶ based on the whole mixed gas is sufficient. For example, when heattreatment was conducted in an atmosphere consisting of N₂ gas containing1% O₂ gas and having a partial O₂ gas pressure of 100 Pa, the sameeffects as the above were obtained. There is no particular upper limiton the introduction amount of the gas comprising oxygen from thestandpoints of the impartation of p-type low resistance and electrodealloying. Any high pressure is usable as long as production is possible.

As shown above, this embodiment provides the following effects. Byforming a current-diffusing electrode combining light transmissionproperties and ohmic properties on a p-type GaN related compoundsemiconductor and further forming thereon a electrode pad containing ametal reactive with nitrogen, not only can the electrode pad beprevented from peeling off, but also the current-diffusing electrode canhave an increased current density and an improved luminance.

The present invention described above relates to light-emitting diodeshaving a light-transmitting electrode and an electrode pad. However, thepresent invention is applicable also to the production of laser diodes(LD), light-receiving devices, and other electronic devices expected toemploy GaN related compound semiconductor devices, such as, e.g. ,high-temperature devices and power devices.

What is claimed is:
 1. A process for producing a p-type GaN relatedcompound semiconductor, comprising: doping a GaN related compoundsemiconductor with a p-type impurity; and subjecting the GaN relatedcompound semiconductor to a heat treatment under a low vacuum conditionof 10 Torr or lower in a gas comprising at least oxygen.
 2. A processfor producing a GaN related compound semiconductor device comprising:doping a GaN related compound semiconductor layer with a p-typeimpurity; forming an electrode on the GaN related compound semiconductorlayer; and subjecting the GaN related compound semiconductor layerhaving the electrode formed thereon to a heat treatment under a lowvacuum condition of 10 Torr or lower in a gas comprising at leastoxygen.
 3. A process for producing a GaN related compound semiconductordevice comprising: doping a first GaN related compound semiconductorlayer with a p-type impurity; doping a second GaN related compoundsemiconductor layer with an n-type impurity; forming a first electrodeon the first GaN related compound semiconductor layer; forming a secondelectrode on the second GaN related compound semiconductor layer; andsubjecting the resultant structure including the first and second GaNrelated compound semiconductor layers and the first and secondelectrodes to a heat treatment under a low vacuum condition of 10 Torror lower in a gas comprising at least oxygen.
 4. The process forproducing a p-type GaN related compound semiconductor acording to claim1, wherein the gas comprising oxygen comprises at least one memberselected from the group consisting of O₂, O₃, CO, CO₂, NO, N₂O, NO₂, andH₂O.
 5. The process for producing a p-type GaN related compoundsemiconductor acording to claim 4, wherein the gas comprising oxygenfurther comprises inert gas.
 6. The process for producing a GaN relatedcompound semiconductor device acording to claim 2, wherein the gascomprising oxygen comprises at least one member selected from the groupconsisting of O₂, O₃, CO, CO₂, NO, N₂O, NO₂, and H₂O.
 7. The process forproducing a GaN related compound semiconductor device acording to claim6, wherein the gas comprising oxygen further comprises inert gas.
 8. Theprocess for producing a GaN related compound semiconductor deviceacording to claim 3, wherein the gas comprising oxygen comprises atleast one member selected from the group consisting of O₂, O₃, CO, CO₂,NO, N₂O, NO₂, and H₂O.
 9. The process for producing a GaN relatedcompound semiconductor device acording to claim 8, wherein the gascomprising oxygen further comprises inert gas.
 10. The process forproducing a p-type GaN related compound semiconductor according to claim1, wherein the heat treatment is conducted at a temperature not lowerthan 400° C.
 11. The process for producing a GaN related compoundsemiconductor device acording to claim 2, wherein the heat treatment isconducted at a temperature not lower than 400° C.
 12. The process forproducing a GaN related compound semiconductor device acording to claim3, wherein the heat treatment is conducted at a temperature not lowerthan 400° C.